Patent GB-670763-A (GB)
Title: Improvements in or relating to control circuits for use in automatic signalling systems
IPC: H04Q3/42
Filed: 1949-10-07
Abstract: 670,763. Automatic exchange systems. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN. Oct. 7, 1949 [Oct. 12, 1948], No. 25785/49. Class 40 (iv). A wiper d sweeps over contacts, those corresponding to idle channels being distinguished from those corresponding to busy channels by having an algebraically lower potential; the wiper d, Fig. 1, is connected to the cathode of a valve B5 whose control grid is connected to the output of a valve B4, so that when valve B4 is conducting, valve B5 is cut-off when wiper d is on busy contacts and conducts only to a limited extent when wiper d is on idle contacts so that the current is insufficient to actuate the relay (not shown) changing the potential of the contacts to the busy value; valve B4 has its control grid connected to the output of valve B5 and to a device responding to the wiper c ganged with wiper d reaching the desired channel; when valve B5 is non-conducting, valve B4 responds to wiper c ganged with wiper d reaching the desired channel by becoming non-conductive, thus rendering valve B5 conducting to actuate the relay (not shown) marking the contact with the busy potential and also to stop the stepping of the wiper d; but when valve B5 is partially conducting, valve B5 is prevented from responding when the wipers c, d reach the desired channel, and the wipers continue stepping. The invention is described with reference to a system in which a wiper MS is switched by dialling signals to one of a bank of contacts distinguished by their direct potential, and a wiper c is stepped over a similar contact-bank until it reaches a corresponding contact representing an idle channel. When wipers MS, c are at the same potential the valves B1, B2, B3 operate as disclosed in Specification 662,215, [Group XL (c)], to increase the potential at point 8 and make valve B4 non-conducting if the channel is free as determined by wiper d. The stepping magnet SM is de-energized when valve B6 is made non-conductive; the control grid of valve B6 is connected to the screen grid of valve B5 so that when valve B5 conducts valve B6 is made non-conducting. To enable breaking-in, a relay (not shown) is energized to change contact s1 and thus vary the potential applied to the control grid of valve B5; when the potentials of wipers MS and c are equal, valve B4 becomes non-conducting and valve B5 conducts to stop the wiper d, in spite of the fact that the contact engaged by wiper d is at the lower potential corresponding to a busy channel. The busy contacts may be marked with potentials indicative of the priority of the connection; thus, trunk connections may correspond to a potential of - 30 volts on the contacts and toll connections to a potential of - 40 volts, so that with contact s1 in its right-hand position, breaking in is possible for toll but not for trunk connections. To break in on a trunk connection, the point 20 may be connected to a voltage source of - 30 volts. More than two degrees of priority may be provided. Fig. 2 shows a system modified for testing. During normal operation, a relay M (not shown) is actuated so that stepping magnet SM is controlled by relay T which is de-energized when valve B4 becomes non-conducting, i.e. when the wipers MS and c are at the same potential and wiper d is on a contact corresponding to an idle channel. For testing, relay M (not shown) is de-energized so that contacts ml ... m7 occupy the positions shown; thus the grids of valves B1, B2 are at the same potential and wiper d is at the potential of an idle channel, so that valve B4 should be non-conducting; if valve B4 conducts, relay T is energized to de-energize alarm relay Al and thus indicate a fault. Contacts m3, m4 short-circuit part of resistances R<SP>1</SP>4, R<SP>1</SP>12 to decrease the threshold value of the voltage - comparing circuit formed by valves B1, B2 so that ageing of these valves is indicated as a fault before it affects the normal operation of the circuit. Specification 666,445, [Group XXIX], also is referred to.